In a conventional system, a main catalytic converter is arranged on the downstream side of an exhaust system, such as below a vehicle body floor. In such a system, a sufficient exhaust purification cannot be expected after a cold start of the internal combustion engine and until the temperature of the catalytic converter rises so that the converter is activated. In addition, the closer to the upstream side of the exhaust system the catalytic converter is, namely to the internal combustion engine side, the more problems there are with decreased durability due to the thermal deterioration of the catalyst of the converter.
Therefore, as disclosed in Japanese Laid Open Patent No. H05-321644, an exhaust system has been proposed in which a bypass is provided in parallel to an upstream side portion of the main path having the main catalytic converter, and another bypass catalytic converter is provided on the bypass, and a switching valve for switching these paths are provided therebetween so that the exhaust is guided to the bypass immediately after a cold start. With this structure, the bypass catalytic converter is positioned on the upstream side of the main catalytic converter in the exhaust system and is activated at a relatively early stage so that exhaust purification can be started from the earlier stage.
According to the conventional exhaust system, the bypass splits from the main path, downstream of the confluence point of the exhaust manifold. In other words, the main path and the bypass are parallel, downstream of the confluence point at which the exhaust paths extending from respective cylinders of a multiple cylinder internal combustion engine are joined together, so that the device becomes large, and in particular, when the bypass catalytic converter is provided close to the internal combustion engine, it is difficult to provide the converter in the engine room of the vehicle.